Universal serial bus (USB) devices are configured to couple to other USB compatible devices using a standardized USB connector. Included in the USB connector is a power source connection, typically denoted as VBUS 1, VBUS or the like, which transfers power between coupled USB devices.
A USB “master” device, when acting as a “host” device, provides power over the VBUS 1 connection to a USB “slave” device. The USB master/host device may provide some or all of the USB slave device power needs from an internal battery in the USB master device, or from an ac adapter source or other alternative power source accessible to the USB master device, depending upon the type of USB device. (Some USB slave devices are not configured to receive power from a USB master/host device because of their large power supply requirements which exceed USB maximum power specifications.) USB standards provide that a USB master/host device provide a +5 volt (V″), 100 milli-amp (mA) power supply, or a +5V, 500 mA power supply, from the USB master device, depending upon the device configurations and the power needs of the USB slave device and the power supply capability of the USB master/host device.
For example, a USB compatible laser printer requires an external power source and is not configured to draw power from a USB master/host device. Other types of printers may have sufficiently low power requirements such that they may be configured to draw power from a USB master/host device.
Some USB master/host devices may only have a limited power supply such that they are able to host (provide power to) USB slave devices requiring no more than a +5 volt (V″), 100 milli-amp (mA) power supply. Such a USB master/host device is not configured to host USB slave devices requiring more than a +5 volt (V″), 100 milli-amp (mA) power supply.
USB devices may be configured to operate as either a USB master, USB master/host and/or a USB slave, depending upon the current operating function of the USB device. When acting as a USB master/host, the USB device provides power to the USB slave device to which it is coupled. When acting as a USB slave, the USB device may receive power from the USB master device to which it is coupled (if the USB device is configured to take power from a host when operating as a slave device).
In situations where a portable USB device is acting as a USB master/host and supplying power to a USB slave from its internal battery, the USB device's power supply (capacity) may be limited. That is, if the USB device and the USB slave device together consume the limited power of the internal battery, both devices will fail when power is used up in the battery. Furthermore, the amount of operating time provided by the internal battery is reduced since the battery simultaneously powers both its USB device (operating in a master/host mode) and the connected USB slave device.
Docking stations may be configured to receive USB compatible devices to facilitate communications between USB devices coupled together via the docking station. In some situations, a USB compatible docking station may itself be coupled to a power source (battery, ac adapter, or other source) such that USB slave devices coupled to the USB docking station receive power from the USB docking station.
However, such docking stations are not configured to power USB devices that may operate as either a USB master or a USB slave since the docking station does not include a processing means to determine the operating mode of the USB device.